Early cardiac pacemakers provided a fixed-rate implantable pulse generator (IPG) whose output could be reset by sensed atrial and/or ventricular depolarizations. Modern pacemakers can be programmed to operate in single or dual chamber modes of operation. They change their pacing rate by delivering pacing stimuli to the atrium and/or ventricle at rates that vary between an upper rate and lower rate limit by tracking the sinus or sensor-indicated rate (SIR).
More recently, single and dual chamber pacemakers have been developed which measure and change their pacing rates in response to a much wider variety of sensors that are directly or indirectly related to metabolic requirements. Such sensors include, among others, QT interval evoked response, physical activity, the change of right ventricular blood pressure over time, venous blood temperature, venous blood oxygen saturation, respiration rate, minute ventilation, and various pre and post-systolic time intervals measured by impedance or pressure sensing within the right ventricle of the heart. These sensors may be used alone or in combination with another sensor(s). Such sensor-driven pacemakers have been developed for the purpose of restoring rate response in patients lacking the ability to increase their cardiac rate adequately during exertion.
One popular method for measuring a patient's demand for oxygenated blood is to monitor the patient's level of physical activity by means of a piezoelectric, microphone-like transducer. A pacemaker which employs such a method is disclosed in U.S. Pat. No. 4,485,813 to Anderson et al. In typical prior art rate-responsive pacemakers, the pacing rate is determined according to the output from an activity sensor.
The cardiac rate, however, is normally controlled by a complex set of inputs to the autonomic nervous system. Consequently, no single sensor has been found to be entirely satisfactory for controlling rate response functions. Some of the shortcomings of single-sensor, rate responsive pacemakers, for example, include: (1) long-term sensor instability, resulting from degradation; (2) long-term changes in correlation between sensor output and how it is measured, due to physiologic changes in the patient, such as biologic/sensor interface changes due to tissue changes; (3) changes in sensor sensitivity; and (4) the need for frequent re-programming to accommodate the foregoing problems, as they are encountered.
To address these problems in single sensor of the prior art, it has been proposed to utilize other physiologically based parameters to assess a patient's metabolic demand. One such parameter is minute ventilation that has been clinically demonstrated to be a parameter that correlates directly to the actual metabolic and physiologic needs of patients and has been combined with activity sensors (piezo-electric or accelerometer) in a single pacemaker.
Thus, there are now several multiple-sensor pacemakers capable of varying their rate from multiple sensor inputs. Unfortunately, a highly reliable and efficient implementation of such multiple sensor-driven rate response has proven to be difficult and, at times, not very satisfactory. In addition to those problems listed above as to single-sensor pacemakers, other problems which are typically encountered include: (1) differences between sensors in long-term stability; (2) differences between sensors in immunity to noise; (3) differences in response time due to changing metabolic conditions; (4) differences in correlating each sensor output and measuring its output; (5) time response lags during rate response optimization process; and (6) complex setup procedures, including the need for frequent re-programming.
Various methods to overcome these multi-sensor issues have been proposed. Typically, these proposals seek to calibrate the sensor input post implant when the patient is at rest or asleep. Many sensors can be used to indicate when the patient is at rest and/or in a sleep state; such sensor signals include the activity level, the activity variance, and possibly the inclination of the patient. See, for example, U.S. Pat. No. 5,626,622, issued to Cooper, entitled “Dual Sensor Rate-Responsive Pacemaker”, which discusses the use of an activity sensor to determine the activity level of the patient. See also, for example, U.S. Pat. No. 5,476,483, to Bornzin et al., entitled “System and Method for Modulating the Base Rate During Sleep for a Rate-responsive Cardiac Pacemaker”, which discusses the use of activity variance to determine if the patient is at rest or sleeping.
Physician intervention during follow-up has also been asserted as a solution. The process of initialization or recalibrating a physiologic sensor post implant involves having the physician program the cardiac pacemaker so that the sensor is appropriately tuned or optimized to allow the cardiac pacemaker to accurately respond to changes in the patient's metabolic demand. The algorithms used to control physiologic sensors typically have relatively long time constants of up to 30 minutes or more. A lengthy time constant is not desirable, however, in that it consumes a substantial amount of clinical time in order to achieve the initialization or recalibration. As will be appreciated, this is also costly and undesirable in that it effectively limits the number of patients whose pacemakers may be initialized within a given period of time so as to negatively impact the efficiency of the clinical operations. Invariably, physicians end up bypassing the lengthy automatic initialization process by manually setting the response slope of the sensors. Manual optimization of the sensors is not the best approach because it is typically based on “best guess” approximation that is often highly subjective and more likely to result in non-optimal sensor rate settings.
Still further drawbacks exist with regard to the algorithms employed to optimize both the physiologic and activity sensors. These algorithms, typically referred to as “automatic slope algorithms,” are used to adapt a sensor response based on a feedback mechanism. One common feedback mechanism is dependent upon whether the pacing rate achieves a maximum sensor rate (MSR) within a predetermined time period. MSR is defined as the maximum pacing rate allowed as a result of sensor control input that typically programmed from 100 to 180 pulses per minute (ppm) in 5 or 10-ppm increments. Another common feedback mechanism is dependent upon whether the pacing rate achieves a target sensor rate (TSR) that is lower than the MSR within a predetermined time period such as 8 days. Algorithms of the first type are known to result in inappropriate response optimization in that it assumes that the patient exercises up to the programmed MSR in every time period. Algorithms of the second type require programming of a patient individual TSR which can be described as the typical maximum daily achieved rate. However, this is an arbitrary rate since the physician will typically rely on subjective patient data to program the rate. Furthermore, both types of algorithms have very long time constants for optimization, typically measured in weeks or months. Such an approach is generally disclosed in U.S. Pat. No. 6,273,856 issued to Sun, et al. This process is contrary to the physician's goal of sending the patient home with an optimized response immediately after implant. Another disadvantage with these algorithms is that they typically result in extremely aggressive sensor response after a period of sedentary behavior or immobility.
An ambulatory solution to these issues has been implemented in many multi-sensor pacemakers, known as sensor cross check. Sensor cross check evaluates the input of the respective sensor and determines whether it is valid and, based on this evaluation, whether it should be used or ignored. For example, a patient with pulmonary dysfunction may be breathing somewhat rapidly and/or deeply. This breathing pattern would trigger the MV sensor to increase the rate, although the activity sensor would indicate the patient is at rest. Generally, a multi-sensor pacemaker will allow a limited increase in the pacing rate that may or may not be appropriate for this particular patient. On the other hand, if a patient is riding in a car over a bumpy road, the activity sensor might call for an increase in rate, the MV sensor would prevent any increase because it would indicate the patient is a rest. In the main, however, sensor cross checks do not provide a sensor mode switch to relieve patient symptoms caused by any inappropriate sensor responses.
A need therefore exists for an improved multi-sensor cardiac pacemaker that will automatically select the appropriate rate response sensor mode (integrated, MV only, Activity only, QT only, etc.) so as to optimize the device's performance and the patient's cardiac rate.